Transformation of exogenous dissolved organic matter sourced from the northwest Pacific in the South China Sea

Mixing and transformation of dissolved organic matter (DOM) in the deep sea are important biogeochemical processes, affecting element cycling and distribution in the ocean. These processes, however, occur in a long time scale and difficult to discern due to the somewhat conservative behavior of DOM in the deep ocean. Here we aim to elucidate these processes by taking advantage of the system of South China Sea (SCS), the largest marginal sea in the tropical northwest Pacific Ocean (NPO), where the Kuroshio current exchanging with the SCS through Luzon Strait (LS). The SCS basin has a three-layer circulation including cyclonic circulation in the upper layer and deep basin, thus it mainly receives water mass intrusion through LS at the depths of both surface (<500m) and deeper ocean (1500-2500m). Our specific goals are to investigate the exchange of exogenous dissolved organic matter (DOM) derived from the NPO as well as the transformation of DOM in the SCS using their radioactive carbon ages and molecular characterizations. DOM was collected at the three stations of NPO, north and central SCS, respectively, using solid phase extraction (SPE). SPE-DOM was characterized by modern mass spectrometry (FT-ICR-MS and ion mobility Q-ToF). The 14C ages of SPE-DOM in the deep sample (2000m) at the SCS basin is 6,300 ±45 years, 130 years older than that in the deep NPO (6170 ±45), which agrees with the reported turn over time of SCS. In other words, the deep SCS can be viewed as a natural “incubator” to observe microbial transformation of DOM at a time scale of ~100 years. Basing on the isopycnal mixing model, we found that the Kuroshio intrusion greatly affected the distribution of dissolved organic carbon (DOC) in the north SCS. The degradation of DOC occurred during the mixing, and the degradation was enhanced by the high nutrient levels in the surface water of north SCS. The FT-ICR-MS and Q-ToF data of SPE-DOM, including isomeric information, are in process and will be reported.